Fuel cells are main power supply source of a fuel cell system, and generate electricity through an oxidation/reduction reaction of hydrogen and oxygen.
Hydrogen of high purity is supplied from a hydrogen storage tank to an anode of a fuel cell stack (hereinafter, referred to as a ‘stack’), and air in the atmosphere supplied by an air compressor or other supply devices is introduced into a cathode of the stack.
An oxidation reaction of hydrogen is undergone in the anode to generate hydrogen ions (e.g., protons) and electrons, and the hydrogen ions and electrons generated in this way move to the cathode through a polymer electrolyte membrane and a separator. Further, a reduction reaction, in which the hydrogen ions and electrons moved from the anode and oxygen in the air supplied by an air supply device participate, is undergone in the cathode so that water is produced and electric energy due to flow of the electrons is generated at the same time.
However, the hydrogen passing through the anode is resupplied to the anode along a hydrogen recirculation line, and nitrogen and other gases included in the air passing through the cathode are crossed over through the polymer electrolyte membrane and introduced into the anode through the polymer electrolyte membrane. Accordingly, as an operation time of the fuel cell system increases, a concentration of the hydrogen in the anode gradually decreases.
When the concentration of the hydrogen in the anode is 70% or more, there is no difficulty in maintaining the performance of the fuel cell system at the highest level. However, if the concentration of the hydrogen of the anode decreases to less than 70%, the performance of the fuel cell system deteriorates. In order to solve this problem, existing fuel cell systems perform a hydrogen purging operation of discharging hydrogen passing through the hydrogen recirculation line and other gases to outside of the fuel cell system to adjust the concentration of the hydrogen in the anode.
Further, when the fuel cell system is stopped, the concentration of the hydrogen in the anode changes according to a time period which has elapsed since the fuel cell system was stopped due to a reaction of residual hydrogen and residual oxygen, cross-over of nitrogen and other gases, introduction of exterior air through a valve or other members. However, when the fuel cell system is started, the existing fuel cell systems collectively performs a hydrogen purging operation without considering a change of the concentration of the hydrogen in the anode according to the stop time period. Accordingly, in the existing fuel cell system, a supply pressure of the hydrogen increases as an excessive hydrogen purging operation is performed when the fuel cell system is started, an amount of the hydrogen exhausted to the outside of the fuel cell system increases, and it is therefore difficult to satisfy regulations of the concentration of hydrogen of exhaust gases.